1. Field of the Invention
The present invention relates to semiconductor devices and their methods of fabrication. More particularly, the present invention relates to the formation of gate stacks. Even more particularly, the present invention relates to forming a gate stack having superior thermal stability and reduced diffusion into silicon-bearing semiconductor structures.
2. Description of the Background Art
Currently, the semiconductor industry has an interest in reducing the critical dimensions of transistors. As such, the thickness of the gate oxide must also be reduced. In so doing, the related art has faced problems associated with a significant increase in direct tunneling leakage current through a very thin gate oxide (i.e., <25 Angstroms). In an effort to suppress the severe gate leakage current, a high dielectric constant (high-k) material may be used as a gate dielectric, replacing a conventional thermal oxide. Several high dielectric constant (high-k) materials (metal oxides) are good candidates for gate dielectric insulators: zirconia or zirconium dioxide (ZrO2), hafnia or hafnium dioxide (HfO2), titania or titanium dioxide (TiO2), tantala or tantalum pentoxide (Ta2O5), and the like.
However, a high-k gate dielectric insulator, such as the foregoing metal oxides, must have a thickness which is much greater than that of a conventional thermal oxide to be similarly effective, because the direct current density is exponentially proportional to a dielectric layer's thickness. Thus, the direct tunneling current flow through a gate dielectric insulator may be significantly reduced, motivating its use in very small transistors. Another major problem with using a high-k material is thermal instability. High-k materials tend to diffuse into the silicon (Si) substrate, a polysilicon (poly-Si) gate, or a polysilicon-germanium (poly-SiGe) gate during subsequent high temperature processing steps. Therefore, a need exists for a method of fabricating a semiconductor device having a high-k dielectric gate insulator with good th mal stability which does not diffuse into the Si substrate, the poly-Si gate, or the poly-SiGe gate when experiencing subsequent high temperature processes.